1. Field of the Invention
The present invention relates to a surgical knife blade of the type primarily intended for making incisions in the eye, as for a clear corneal incision. The cutting edges of the blade are defined by the intersection of bevels formed on the blade's anterior surface and posterior surface, with the plane of the bevels with respect to the anterior and posterior surfaces being non-symmetrical.
2. Description of the Prior Art
Numerous prior art devices and blades are known for making incisions in the eye. Since the cornea and sclera are spherical, any blade penetrating their surface at an angle other than 90.degree. will produce an irregular, or curved, incision line resembling a "smile." Ophthalmic surgeons have struggled with this problem for years, since curvilinear incisions are less likely to reapproximate as quickly, are less efficient, and are much less likely to create a suture-less water-tight seal. In an attempt to create substantially linear, perpendicular incisions, surgeons find themselves applanating the globe of the eye in an attempt to flatten it out, and invariably "dimple down" as soon as the tip of the blade reaches Descemet's membrane. However, problems are created with this "dimpling down" procedure in that this requires the surgeon to lift the back of the blade upwardly in order to point the tip of the blade downward. This maneuver causes the tissue in the corners of the external incision to tip, compromising its water-tight integrity and creating stromal distortion in the path of the tunnel. Lifting the back of the blade also increases the angle of the cut, making it less tangential to the circumferential arc of the cornea. This results in a reduced valve sealing surface area, further comprising water-tight integrity. In an attempt to compensate for this tearing of tissue, surgeons hydrate the corners of the incision. When a conventional flat blade enters a spherical object at an angle, as when making a clear corneal incision, it is the fact that the shoulders of the blade enter the globe closer to the center than the tip that produces the curved, "smile" incision.
While the phenomenon described above is certainly well-known and recognized when one attempts to incise the surface of a spherical member such as the eye, the producing of a curved incision derives most directly from the fact that state-of-the-art blades are typically symmetrical when the anterior surface is compared to the posterior surface. For example, U.S. Pat. No. 5,376,099 to Ellis, et at., discloses an undercut diamond surgical blade. However, when the bevels forming the cutting edges on the anterior surface of the blade are compared with the bevels on the posterior surface of the blade, one immediately recognizes that the bevels are identical. A similar construction is taught in U.S. Pat. No. 5,336,235 to Myers, even though the blade of that invention is slightly curved with respect to its major longitudinal axis.
The ophthalmologic surgical instrument disclosed in U.S. Pat. No. 4,688,570 to Kramer, et al., teaches the use of a cutting blade which is also quite symmetrical when the bevels of one surface of the blade are compared to the bevels on the other surface, those bevels defining the blade's cutting edge.
A variety of blade configurations are disclosed in U.S. Pat. No. 5,098,438 to Siepser, but the blades of that patent have bevels on only one of the blade surfaces.
While the blade disclosed in U.S. Pat. No. 5,201,747 to Mastel is shown as having three cutting edges, the bevels defining those cutting edges on the opposed surfaces of the blade are identical.
The blades disclosed in U.S. Pat. No. 5,203,865 to Siepser are virtually identical to those disclosed in the Siepser '438 patent.
A surgical blade similar to the Siepser blades in that bevels are provided on only one blade surface is taught in U.S. Pat. No. 5,370,652 to Kellan.
Finally, U.S. Pat. No. 5,217,476 to Wishinsky and U.S. Pat. No. 5,224,950 to Prywes each disclose surgical knife blades primarily for use in eye surgery wherein the cutting edges are defined by identical bevels on both surfaces of their respective blades.
It is, therefore, clear that there remains a great need in the art for a surgical knife blade capable of making a straight, linear incision to create a self-sealing clear corneal incision without having to "dimple down" into Descemet's membrane, with the increased risk of tearing tissue at the edges of the incision as is almost always encountered using the blades available today.